An RF signal input circuit of a UHF tuner usually includes an impedance transformation network and a tuned circuit connected in cascade between a UHF antenna input and the input of an RF amplifier.
The impedance transformation network is the interface between the different impedances presented at the antenna and RF amplifier inputs and transforms or converts the impedance presented at the antenna input to an impedance more closely resembling the impedance presented at the input of the RF amplifier. For example, a balanced transmission line, usually known as "twin-lead" transmission line, is often used to couple RF signals received by a UHF antenna to the UHF tuner of a television receiver. The UHF tuner has two terminals to which the conductors of the "twin-lead" transmission line are to be connected. The "twin-lead" transmission line has a balanced impedance configuration with respect to ground, but the RF amplifier of the tuner has an "unbalanced" impedance configuration with respect to ground. An impedance transformation network, commonly referred to as a "balun" is used to convert the balanced impedance configuration presented at the UHF antenna input terminals to the unbalanced impedance configuration presented at the input of RF amplifier. The balun may comprise a transformer with its primary connected between the UHF antenna input terminals and its secondary connected between the input of the RF amplifier and signal ground. Often the primary comprises a coil including a few turns of wire and the secondary comprises a conductor strip located to be magnetically coupled to the coil.
The tuned circuit of the tuner has a frequency selective response which is controlled to couple the RF signal received from the antenna which corresponds to a channel to be tuned to the input of the RF amplifier while rejecting unwanted signals. The tuned circuit includes an inductance element and a variable capacitance element. The inductance element may be part of the impedance transformation network such as the secondary of a balun transformer. Typically, the variable capacitance element is a "varactor" diode which is back-biased by a tuning voltage to exhibit a capacitance which changes in inverse relationship with changes in the magnitude of the tuning voltage.
A dual-gate field-effect transistor (FET) is often used as the RF amplifier because it amplifies with relatively little distortion and because it can be readily gain-controlled by the application of a gain-control signal to its second gate electrode.
One type of well known UHF RF signal input circuit is described in U.S. Pat. No. 4,048,598 issued to S. P. Knioht on Sept. 13, 1977. It employs a balun transformer, a parallel-tuned circuit in which the secondary of a balun transformer is connected in parallel with a varactor diode, and a dual gate FET. The parallel-tuned circuit tends to have a low-impedance compared with the impedance of the FET at the low frequency end of the UHF range. An impedance transformation network, including a series-connected capacitor and a shunt-connected inductor, is coupled between the parallel tuned circuit and the FET to increase the effective impedance presented by the parallel tuned circuit at the low frequency end of the UHF range. The impedance transformation network tends to improve the power gain of the UHF RF signal input circuit without substantially degrading the power gain at the upper frequency end of the UHF range.
The bandwidth of a parallel-tuned circuit, such as employed in the Knight type of UHF RF signal input circuit, tends to increase as a function of frequency. Theoretically, the bandwidth of an ideal parallel-tuned circuit tends to vary as the square of the ratio between the upper and lower frequencies of interest. Thus, for the frequency range of UHF channels employed in the United States (i.e., channels 14-70), having an upper frequency limit of about 800 MHz and a lower frequency limit of about 470 MHz, the bandwidth of a parallel-tuned circuit can theoretically change by a factor of about 3. For example, assuming the bandwidth of a parallel-tuned circuit used in a UHF tuner has been constrained to 30 MHz at the lower frequency end of the UHF range, the bandwidth at the frequency upper end of the UHF range can be 90 MHz. As a result, the ability to reject unwanted signal at the upper end of the UHF range can be considerably impaired.
The bandwidth of a series-tuned circuit varies significantly less as a function of frequency compared with the bandwidth of a parallel-tuned circuit. Theoretically, the bandwidth of an ideal series-tuned circuit does not vary as a function of frequency.
One type of UHF input circuit utilizing a primarily series-tuned circuit is employed in the 2003 television tuner manufactured by Deutshe Thomson Brandt. It includes first and second inductors and a varactor diode connected in series, in the order named, between signal ground and the input of a FET RF amplifier. The antenna input is connected to the junction of the first and second inductors. The inductors comprise an impedance "step up" network (i.e., the RF voltage presented to the varactor diode is higher than that applied to the junction of the inductors.) A capacitor is connected in shunt with the input of the amplifier and forms a variable capacitive voltage divider with the varactor diode. The capacitive voltage divider is a variable impedance transformation network which tends toward making the relationship the impedance presented by series-tuned circuit and the impedance presented by the amplifier relatively uniform (compared to the situation without the shunt capacitor) as a function of frequency. This improves the uniformity of such parameters as power transfer and noise performance. This UHF RF signal input circuit can be called a "split-capacitor" arrangement due to the capacitive voltage divider configuration.
Unfortunately, the shunt capacitor of a "split-capacitor" RF signal input circuit is connected across the series-tuned circuit comprising the series combination of the inductor and varactor diodes and results in a tuned circuit having both series and parallel capacitance elements. This results in a bandwidth variation as a function of frequency. Nevertheless, the bandwidth of the primarily series-tuned circuit of the "split-capacitor" UHF RF signal input circuit is still more uniform as a function of frequency than that of a purely parallel-tuned circuit.